docs: def providerIsAuthoritative(providerID, canonicalID):
      """Is this provider ID authoritative for this XRI?
  
      @returntype: bool
      """
      # XXX: can't use rsplit until we require python >= 2.4.
      lastbang = canonicalID.rindex('!')
      parent = canonicalID[:lastbang]
      return parent == providerID